Windows can be secured using thumb screw locks that prevent the glass from sliding open on the track. If you don't have the screw locks consider some sturdy one inch dowels that are cut to just fit into the sliding tracks.
Walk around your house from the outside and look for any potential problem areas for break-ins. Remove bushes or trees that obscure the house from the road, or would provide cover for someone trying to get into your home. Secure ladders in a locked area to prevent easy access to upper windows and keep all tools secured in a locked area.
Avoid hiding keys in outside areas, they are never a secret or secure as you may think. Instead, consider leaving a key with a trusted neighbor, coworker or friend that you could contact should you accidentally lock yourself out of your house.
